Nitrocellulose, which is processed into various types of propellants, is manufactured from cellulose feed stocks comprising fine fibers of cotton linters or macerated wood pulp.
The present methods used to manufacture nitrocellulose involve the nitration of cellulose with nitric acid, sulfuric acid and water. This process requires a purification operation. The method of purification of cellulose by the military has been relatively unchanged since World War I. The process includes neutralization and stabilization steps which involve an acid boiling operation (stabilization), requiring 25-50 hours, a cutting operation, a poaching operation requiring eight hours, a washing operation and a screening operation. The acid boiling and poaching purification operation consumes large quantities of water, steam and electricty. Although the resulting product is satisfactory, the purification is long and costly. The acid boiling and poaching operations in addition require a large amount of floor space. Several European countries use batch autoclaves to reduce purification time. This approach requires heating up and cooling down cycles, thus consuming considerable amounts of energy. Continuous digesters (tube cookers) have been employed for purifying nitrocellulose but have the following disadvantages:
(a) Inadequate agitation; PA1 (b) No means of venting gases produced in the purification; PA1 (c) Requires cuttiing prior to stabilization; and PA1 (d) Depends on long runs of tube cooker piping.
Lack of proper agitation causes line blockage, a hazardous condition. Maintenance of turbulent flow through tube cookers at temperatures from room temperature to operating temperatures can be a problem because the viscosity of the material being purified varies with temperature.
Retention of gaseous carbon dioxide, nitric oxide and nitrous oxide in the purification medium, is believed to have an adverse effect on the suitability of the nitrocellulose for military uses. Presently used processes for purifying nitrocellulose to meet military specifications remove the unwanted gases by either boiling or venting.
Cutting nitrocellulose prior to the acid boiling step (stabilization) to reduce the probability of line blockage extends purification time. Laboratory studies have shown that the stability of the nitrocellulose is reduced if the nitrocellulose is beaten (cut) before the acid boil operation.
There are several ways in which the proper retention or cooking time can be achieved, for example, in some tube cooker designs, long runs of piping are used. These long runs are susceptible to line blockage which, at the elevated operating temperatures of tube cookers, can be hazardous. High pressure flushing of such blocked tubes is suspect since this might result in the blockage being pushed further down the tube with a probability of cook-off.
The above demonstrates that there is a need for an apparatus and process which will permit continuous stabilization and purification of nitrocellulose to meet military specifications economically, without the above described problems and disadvantages.